You can go to Edit > Backdrop, and load your image, or you can simply load a plane Primitive, and apply the image as a texture in the Surfaces pane in the Diffuse channel. It really depends on what you want to do?
One small gotcha if you use the backdrop setting — the pic you use should be as close as you can get to the same proportions as your final render. It doesn’t have to be exactly the same pixel dimensions, as long as the ratio of width to height is similar. If it isn’t, then DAZ|Studio will make it fit — with results varying from hilarious to ridiculous.
What you can do, is to make the render large enough for printing at a certain DPI resolution. For exaplle, if you want a 10 inch x 8 inch print at 300 DPI, you would set the render size to 3000 x 2400 pixels. The size in inches multiplied by the DPI.